Sediment accumulation in reservoirs can undermine both the safety and utility of water storage facilities, and can have severe ecological consequences for inundated areas. Yet we know surprisingly little about how these areas recover when reservoir levels recede, an occurrence that will only become more commonplace in the face of global climate change and reduced snowmelt runoff.
Lake Powell Reservoir, which sits behind Glen Canyon Dam, filled for the first time in 1980 and has been steadily receding since about 2000. At full capacity, the reservoir is the second-largest by capacity in the U.S. and creates a slack water impoundment extending more than 180 miles upstream. Lake Powell inundates the lower reaches of numerous tributary canyons flowing into the Colorado River, along with drowning the lower 18 rapids of Cataract Canyon on the Colorado River.
In collaboration with Utah State's Center for Colorado River Studies, and recreational river users, I am beginning to examine how tributary canyons and the rapids of lower Cataract Canyon are recovering following the recession of water in Lake Powell. Using recently-collected geospatial data including airborne lidar and multibeam sonar, we’re examining the thickness and accumulation rates of sediment in tributary canyons around the Lake.
Estimated sediment accumulation rates over the 56 year period between 1959 and 2019 in the lower (downstream) portion of Lake Powell Reservoir. Pre-dam topography was collected by photogrammetric interpretation of aerial photographs, while 2015 bathymetry was collected by the U.S. Geological Survey using multibeam sonar. The difference in elevation between the two datasets provides an estimate of sediment accumulaton during the post-dam period.
We’re also using airborne lidar to map the water level drops through rapids of Lower Cataract Canyon, which can be used in future surveys to track the progression of rapid re-emergence via downcutting of the channel bed through accumulated sediment.
Because airborne lidar data provides an estimate of water surface elevation, we can use this information to quantify hydraulic drops through rapids in lower Cataract Canyon and track the progression of these metrics as once-inundated rapids return to the river. Shown here are two rapids that are at different stages of recovery, Rapid #28 (left) and Gypsum Canyon Rapid (right).
While just in its early stages, the results of this work will not only provide information on how rapidly riverine ecosystems recover from severe disturbance, but information gained will also enable the targeted management of reservoir levels for multiple uses, including water storage, recreation, power generation, and preservation of historic and cultural sites.